In a data buffering system illustrated in FIG. 1, a dual port data buffer 10 such as a first in—first out (FIFO) interfaces an input device 12 by an input bus 14 and an output device 16 by an output bus 18. By monitoring the buffer write operations through the write control line and the write clock line and the buffer read operations through the read control line and the read clock line, a buffer occupancy monitoring unit 20 determines the occupancy of the buffer. Then, a buffer management logic unit 22 uses the buffer occupancy information provided by the buffer occupancy monitoring unit to grant data write or data read to output device 16 respectively.
When data write is granted, the data are written into the data buffer by means of elementary transfers cadenced by the write clock, each transfer carrying an amount of data referred to the input logical unit corresponding generally to an integer number of bytes and equal to the input bus width.
Likewise, when data read is granted, the data are read from the data buffer 10 by means of elementary transfers cadenced by the read clock, each transfer carrying an output logical unit corresponding generally to an integer number of bytes equal to the output bus width.
In many applications, especially in telecom systems, the data following through the data buffer is often packetized and consists in fixed length data packets. Unless the input and output devices have their own buffering area for packet re-assembly and disassembly, the buffer is accessed by writing and/or reading in a row insofar as many logical units are needed to transfer an entire packet.
As the transfer of a packet cannot be interrupted once started, this implies for the buffer management logic unit 22 to implement a protection against buffer overrun by de-granting packet write when the amount of free room available in the data buffer becomes lower that needed to store a number of entire packets that depends on the input device latency to grant, and to implement a protection against buffer underrun by granting packet read when enough data are available in the data buffer, this allowing to read an entire packet without facing a buffer empty condition when the read clock is faster than the write clock.
As far as underrun is concerned, the most common protection technique consists in asserting read grant whenever the contents of the data buffer are equal or higher than one entire packet. Whenever the data latency in the buffer needs to be minimized, a “cuthrough” mechanism is implemented. It grants reading when the minimum amount of data, safe against underrun, resides in the buffer and therefore allows to start reading while buffer contains less than an entire packet. This is generally achieved by asserting a read grant whenever the buffer contents are higher than a predetermined threshold which is either hardware or provided to the buffer management logic by means of a configuration register. Unfortunately, the determination of a threshold value requires an exact knowledge of the read and write frequencies prior to hardwire it or to configure the buffer parameters, which is rarely the case in view of the possible frequency fluctuations.